The present invention relates generally to filters for use inside blood vessels. More particularly, the present invention relates to thrombus filters which can be securely affixed at a selected location in the vascular system and removed when no longer required.
There are a number of situations in the practice of medicine when it becomes desirable for a physician to place a filter in the vascular system of a patient. One of the most common applications for vascular filters is the treatment of Deep Venous Thrombosis (DVT). Deep Venous Thrombosis patients experience clotting of blood in the large veins of the lower portions of the body. These patients are constantly at risk of a clot breaking free and traveling via the inferior vena cava to the heart and lungs. This process is known as pulmonary embolization. Pulmonary embolization can frequently be fatal, for example when a large blood clot interferes with the life-sustaining pumping action of the heart. If a blood clot passes through the heart it will be pumped into the lungs and may cause a blockage in the pulmonary arteries. A blockage of this type in the lungs will interfere with the oxygenation of the blood causing shock or death.
Pulmonary embolization may be successfully prevented by the appropriate placement of a thrombus filter in the vascular system of a patient""s body. Placement of the filter may be accomplished by performing a laparotomy with the patient under general anesthesia. However, intravenous insertion is often the preferred method of placing a thrombus filter in a patient""s vascular system.
Intravenous insertion of a thrombus filter is less invasive and it requires only a local anesthetic. In this procedure, the thrombus filter is collapsed within a delivery catheter. The delivery catheter is introduced into the patients vascular system at a point which is convenient to the physician. The delivery catheter is then fed further into the vascular system until it reaches a desirable location for filter placement. The thrombus filter is then released into the blood vessel from the delivery catheter.
In the treatment of Deep Venous Thrombosis, a thrombus filter is placed in the inferior vena cava of a patient. The inferior vena cava is a large vessel which returns blood to the heart from the lower part of the body. The inferior vena cava may be accessed through the patient""s femoral or jugular vein.
Thrombus filters may be placed in other locations when treating conditions other than deep venous thrombosis. For example, if blood clots are expected to approach the heart and lungs from the upper portion of the body, a thrombus filter may be positioned in the superior vena cava. The superior vena cava is a large vessel which returns blood to the heart from the upper part of the body. The superior vena cava may also be accessed through the jugular vein or femoral vein.
Once placed inside a blood vessel, a thrombus filter acts to catch and hold blood clots. The flow of blood around the captured clots allows the body""s lysing process to dissolve the clots.
It is recognized in the art that it is undesirable for a thrombus filter to change position once it has been place in the desired position by a physician. If a filter becomes loose in the lumen of a blood vessel, it may migrate to a position where it may be ineffective at capturing thrombi. Alternately, and more seriously, a loose thrombus filter may migrate to a dangerous or life threatening position. Prior art filters have addressed this concern by including anchor members which penetrate the vessel walls.
The walls of the blood vessels are lined with a thin inner membrane which may be referred to as the intima or the endothelium. When this inner membrane is disrupted by a foreign object such as a thrombus filter the body responds in a process referred to as neointimal hyperplasia. As a result, the disrupted area of inner membrane is overgrown with a number of new cells. The anchor portions of the thrombus filter are encapsulated with new cell growth, sometimes referred to as endothelial growth.
Due to endothelial growth, thrombus filters placed in the blood vessel of patient become affixed to the blood vessel walls within two weeks after being implanted. Because the portions of the filter contacting the blood vessel wall become fixed in this way, many prior art filters cannot be removed percutaneously after being in place for more than two weeks.
The present invention pertains to a thrombus filter and a method of removing a thrombus filter using minimally invasive methods, and avoiding complications due to endothelial growth. The thrombus filter includes a body member and a plurality of wires. Each wire has a joined end and free end. The joined end of each wire is fixably attached to the distal portion of the body member. Each wire radiates away from the body member along a generally helical path of expanding diameter. The shape of each wire may be generally described as a spiral or helix of expanding diameter. The wires radiate away from the body member to form a generally conical filtering portion which includes a plurality of open cells defined by the wires of the thrombus filter.
The open cells allow blood to flow through the thrombus filter while the wires enable the filtering portion of the thrombus filter to trap or capture blood clots. The generally conical shape of the filtering portion of the thrombus filter urges blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body""s natural lysing process to dissolve the clots.
Each wire extends beyond the filtering portion into a wall engaging portion. The wall engaging portion applies an outward force on the wall of the blood vessel. The body member of the thrombus filter is held in a position proximate the center of the blood vessel by the plurality of wire which engage the blood vessel walls with opposing force vectors. When the wires contact the walls of the blood vessel, they can deform to the generally cylindrical shape of the blood vessel lumen. Thus, the wall engaging portion of the thrombus filter is generally cylindrical in shape when it is positioned in a blood vessel.
Once the thrombus filter has been placed in the desired position by a physician it is undesirable for the thrombus filter to migrate to another position in the vasculature of the patient. If a filter becomes loose in the lumen of a blood vessel, it may migrate to a position where it does not effectively capture thrombi. Alternately, and more seriously, a loose thrombus filter may migrate to a dangerous or life threatening position. As described above, the wires of the thrombus filter are spring biased outward so that they exert an outward force on the walls of the blood vessel proximate the wall engaging portion of the thrombus filter. The outward force applied to the walls of the blood vessel helps prevent the thrombus filter from leaving the desired position.
As described previously, each wire is generally helical or spiraled in shape. The shape of the wires causes them to travel across the wall of the blood vessel at an acute angle relative to the longitudinal axis of the blood vessel lumen. The cross ways engagement of the wires with the wall of the blood vessel also helps to retain the thrombus filter in the desired position.
The wires of the thrombus filter engage the walls along a significant portion of their length. This significant length of engagement between each wire and the walls of the blood vessel also serves to retain the thrombus filter in the desired position, preventing it from migrating along the length of the blood vessel. The relatively large area of contact between the wire and the blood vessel wall serves to minimize disruption to the endothelium or intima portion of the blood vessel. Minimizing the disruption to the endothelium serves to minimize the amount of endothelial growth resulting from the presence of the thrombus filter in the lumen of the blood vessel. Minimizing endothelial growth makes the removal of the thrombus filter less problematic. However, the thrombus filter may be removed even in cases where endothelial growth has occurred.
It is a desirable feature of this thrombus filter that the wires be shaped so that they can be easily pulled through encapsulating endothelial growth if such growth occurs. In a currently preferred embodiment, the cross sectional dimensions of the wires are substantially unchanged along their entire length. In an alternate embodiment, the wires may be tapered so that each free end is generally smaller than other portions of the wire. The shape of each wire proximate its free end aids in pulling the wire through any endothelial growth which may occur.
As described previously, each wire is generally in the shape of helix with an expanding diameter. The gently curved shape of the helix also aids in pulling the wires through any endothelial growth which may occur.
Although the thrombus filter is retained securely in place as described above, it may be removed using minimally invasive methods when such removal becomes desirable. The design of this thrombus filter allows it to be removed using minimally invasive methods while avoiding complications due to endothelial growth. When removal of the thrombus filter is desired, a catheter including a lumen is positioned in the blood vessel. The distal end of the catheter is positioned proximate the thrombus filter, and the proximal end of the catheter extends outside the patient""s body. An elongate retrieval member is positioned in the lumen of the catheter. A mechanical link is formed between the distal end of the retrieval member and the thrombus filter. A proximal end of the elongate retrieval member protrudes beyond the proximal end of the catheter. After a mechanical link is formed between the retrieval member and the thrombus filter, the thrombus filter may be pulled in the lumen of the catheter by applying a twisting and pulling force to the proximate end of the retrieval member. This pulling and twisting force is transferred via the retrieval member to the thrombus filter, xe2x80x9cunscrewingxe2x80x9d it from the endothelial growth.
Pulling the thrombus filter into the lumen of the catheter causes the wires to collapse. The collapse of the wires causes the thrombus filter to assume the general shape of the lumen of the catheter. Once the thrombus filter is pulled into the lumen of the retrieval catheter, the removal of the thrombus filter from the patient""s body becomes a simple matter of withdrawing the catheter from the lumen of the blood vessel.